There is known a polymer compound, which is generally called boron nitride, having a recurring structure of a boron/nitrogen bond.
This boron nitride has a high melting point and excellent electrical, mechanical and chemical properties. Thus it is expected to be available for various uses including a reinforcement for composite materials, the diaphragm of a molten salt battery, an IC base plate and a crucible.
A known process for producing boron nitride fiber comprises heating B,B,B-triamino-N,N,N-triphenylborazine (H.sub.2 NBN--C.sub.6 H.sub.5).sub.3 to 250.degree. C. under a nitrogen atmosphere to thereby give a precursor, a condensed polymer of borazine, which would begin to soften at 150.degree. C. to 200.degree. C.; melting the obtained precursor by heating; spinning the molten precursor; and then slowly heating the precursor fiber thus obtained to a temperature higher than 1000.degree. C. under a nitrogen or an ammonia atmosphere (JP-78-37837, examined Japanese Patent Publication). Other shaped boron nitride products, for example thin film, may be produced in a similar manner from the precursor.
However the shaped boron nitride product obtained by the above process is not purely white but slightly colored in gray or pale gray.
This coloration seems to be caused by the incomplete removal of phenyl groups bound to nitrogen atoms.